BIVALVIA. 81 



Spec. Char. A. testd variabilis plerumque oblongd, angustatd, subtetragond, sub- 

 obliqud, valde incequilaterali, striata; umbonibus distantibus, recurvis ; siphoni-regione 

 productd, biangulatd ; striis radiantibus, numerosis, squamoso-granulatis. 



Shell variable, for the most part elongately oblong ; somewhat oblique, very inequi- 

 lateral; umbones distant, recurved; siphonal region much the larger, biangulated; striae 

 numerous, granulated, and slightly imbricated. 



Length, 3 inches ; height, 1 inch. 



Localities. Bracklesham, Selsey ; var. j3, Barton. 



France, Grignon, Courtagnon, Senlis, Valmondois (Desh.). 



The ligamental area in this species has a broad, flat, and deep depression, with chevron- 

 formed lines, which when the valves are united form lozenge-shaped ornaments imme- 

 diately beneath the umbo, one within the other, the smaller closely approaching the 

 hinge-margin ; sometimes there is another set of the lozenge-shaped marks on the siphonal 

 region, but these latter are more often only parallels to the radiating umbonal lines. In 

 specimens from Barton this area is tinged with a dark-red colour by the remains of the 

 ligamental connector. The hinge-margin is furnished with numerous small teeth, rather 

 more strongly displayed in the young state than in the old. Most of the large specimens 

 from Bracklesham have the outside much eroded and nearly smooth, like specimens of 

 A. tetragona that have inhabited some crypt where the movements of the animal have 

 abraded the otherwise striated surface of the shell. This species is particularly abundant 

 at Bracklesham, where it attains to large proportions. In the full-grown shell from that 

 locality there is generally a large sinus, the ventral margin being deeply indented; but in 

 the younger shells from the same locality this is not so strongly marked, and in some of 

 those from Barton the valves are quite closed ; it is so, however, with various specimens 

 of A. tetragona. The shell called A. biangula, from the Bordeaux and Touraine beds, is by 

 M. Deshayes considered as not identical with the Paris Basin species, and in this I think 

 he is correct. There is, however, a slight difference between our own shell and that of the 

 Paris Basin, ours being more finely striated all over, especially so upon the pedal region. 

 Mr. Sowerby has retained the name of A. Branderi for the British fossil, which, he 

 says (p. 169), "differs from A. biangula, Desh., in having a much less acute keel." The 

 keel, however, is variable in that respect among the French fossils, and this difference does 

 not always exist in the British shell. The pedal region in specimens from Barton is some- 

 times broader or higher than in those from Bracklesham ; this arises from a less sinuated 

 margin, which otherwise contracts the shell at that part ; the large rays have generally a 

 smaller intermediate one, and these are decussated by very visible lines of growth. The 

 Barton shell may, perhaps, be considered as a variety under the name Branderi, but I think 

 there is not a specific difference ; it much resembles A. miniata, Desh. 



